1. Field of the Invention
This invention relates to the field of pressure control systems to control the exhaust pressure in exhausters or exhaust apparatus.
Description of the Prior Art
In oxidation, diffusion or CVD devices or similar devices, after various gases are supplied into the chamber for reaction, the gases which have passed through the reaction process are exhausted by the exhauster.
In this process, because a sudden change in the internal pressure of the chamber might bring an adverse effect on the reaction process in the chamber, a pressure control system is provided in the exhauster so that a constant exhaust gas pressure is maintained at any time, regardless of any change in the flow rate of the gas being exhausted and any change in the pressure in the downstream of the exhauster.
One example of this kind of exhauster pressure control system is the one defined in the U.S. Pat. No. 5,000,221. As shown in FIG. 7, in the pressure control system 100 according to that specification, a piston 300 is provided in the inside space of the main body in which an inlet port 220 and an outlet port 240 are formed; the piston 300 moves back and forth in the space according to variation in the pressure at the inlet port 220 and outlet port 240 to make the fluid pressure at the inlet port 220 constant.
Moreover, other examples of such pressure control systems are those defined in the unexamined Japanese Patent Publications Nos. 150938/92 and 317919/95. As shown in FIG. 8, the pressure control system 102 as defined there turns the valve disc 302 provided between the inlet port 222 and the outlet port 242 to open or close the fluid passage whose cross section is circular. By monitoring the pressure at the inlet port 222 and feeding back the output to the servo motor 400, the valve disc 302 is turned so as to adjust its opening.
FIG. 9 shows the characteristic of the pressure control system 100 as shown in FIG. 7. In this pressure control system 100, which is not a system that detects the pressure at the inlet port 220 and feeds back the detected pressure to operate the piston 300, the position of the piston 300 varies depending on the flow rate. Pressure variation caused by flow rate changes is considerable as shown in FIG. 9, resulting in poor stability. Even when stability is achieved, there remain problems relating to shift and hysteresis.
FIG. 10 shows the characteristic of the pressure control system as shown in FIG. 8. In this pressure control system 102, the pressure at the inlet port 222 as detected by the pressure sensor is fed back to the servo motor 400 to turn the valve disc 300. The accuracy in the control pressure is comparatively high, but, as shown in FIG. 10, the magnitude of pressure variation caused by flow rate changes is considerable, resulting in poor stability.